Can't install a Debian 10 virtual system through netboot

[Fred Boiteux <fbpros@free.fr>]

    Hello,

I'm trying to install a Debian 10.1 on a virtual machine (qemu) running 
on a host without Internet access, using the neboot system. I've setup 
net ressources to allow network boot on the VM, getting a grub menu 
where I can select Debian 10 installation. The VM then downloads linux 
and initrd.gz I've got from netboot.tar.gz (from here : 
http://ftp.nl.debian.org/debian/dists/buster/main/installer-amd64/current/images/netboot/netboot.tar.gz).

I'm using a preseed file, where I've configured to use a local HTP 
mirror (a whole copy of debian-installer ISO : first using netinst 
image, then DVD-1 image, as the CD images aren't available : 
https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/ is also 
serving netinst images, not full CD images...).

The VM boots, starts debian-installer, which configures its network, 
then load additional components, but fail finding my [virtual] disk ! 
When going on the installed system, I can see that virtio_blk module 
isn't loaded nor available ! I've tried to find if a 
*virtio-modules*.deb package was missing, but no such package seems to 
exist for Debian 10 Buster.

Do you have any hint about how to solve my disk detection issue ?

    Thanks,

        Fred.